This invention relates to a method of etching a semiconductor layer such as of a gallium-nitride based compound semiconductor, and a method of manufacturing a semiconductor light emitting device.
Conventionally, semiconductor light emitting devices, such as light emitting diodes (hereinafter called LEDs) and laser diodes, are manufactured by forming a compound semiconductor layer to provide a light emitting layer. Where performing dry etching on part of the formed semiconductor layer during manufacturing such a device, a mask with an opening is formed to carry out etching the semiconductor layer exposed by the opening through a reactive ion etching (RIE) process or CAIBE (Chemical-Assisted Ion Beam Etching) process.
The conventional method of etching a GaAs-based compound semiconductor layer formed on a GaAs substrate is carried out by using a three-layer resist process. That is, a phtoresist hard-baked layer 22 is formed in a thickness of approximately 1 .mu.m on a GaAs semiconductor layer 21 (wherein a photoresist material is applied and baked at approximately 200.degree. C. ), as shown in FIG. 3(a). A Ti film 23 is then formed on the hard-baked layer 22 by sputtering or the like to a thickness of approximately 1000 angstroms. A photoresist layer 24 is further formed to a thickness of approximately 1.5 .mu.m on the Ti film 23. The photoresist layer 24 is patterned to provide an opening at a location to perform etching by a usual photolithographic process.
Then, the Ti film 23 is etched by an RIE process utilizing the patterned photoresist layer 24 as a mask to thereby provide an opening in the Ti film 23, as shown in FIG. 3(b). Thereafter, the photoresist layer 24 and the photoresist hard-baked layer 22 at an exposed portion are removed by an oxygen plasma to thereby expose a portion to be etched of the GaAs semiconductor layer 21, as shown in FIG. 3(c). Then, the GaAs semiconductor layer 21 is etched by a CAIBE process utilizing the Ti film 23 as a mask, as shown in FIG. 3(d). Thereafter, the photoesist hard-baked layer 22 is removed by a solvent such as an NaOH solution, removing the Ti film 23 at the same time. Thus, a GaAs semiconductor layer 21 is provided that is etched at required portion thereof.
Where etching a GaAs compound semiconductor as stated above by the CAIBE process, the selective etch ratio can be taken great for Ti and GaAs. Accordingly, the GaAs layer can be etched to a sufficient extent by using Ti as a mask. However, where etching a recently-developed gallium-nitride based compound semiconductor such as GaN, it is difficult for GaN to provide a great selective etch ratio with respect to Ti, different from the case of GaAs. That is, the selective etch ratio for GaN is given about a 1/3 or less of the GaAs case. This requires a thickness of a Ti-mask film of approximately 2.5 .mu.m in order to etch, for example, GaN by approximately 5 .mu.m. However, a Ti film, when its thickness exceeds 0.2 .mu.m or thicker, worsens in adhesion to a photoresist hard-baked layer, resulting in stripped off. Thus, there is a problem of difficulty for a Ti mask to employ as a mask for etching a gallium-nitride based compound semiconductor.
Meanwhile, where performing etching using a photoresist layer as a mask, since the photoresist layer is further lower in selective etch ratio as compared to Ti. Accordingly, there is a necessity of increasing the mask thickness or reapplying a phtoresist layer in the course of etching. If the photoresist layer is formed thick, the verticality lowers at an opening, resulting in a tapered form. Thus, there is a problem of difficulty in vertically etching the gallium-nitride based compound semiconductor layer.